This invention relates to a paired optically variable article with paired optical structures and inks, paints and foils incorporating the same and a method.
Color from interference thin films is found in nature in fish scales, mother of pearl, etc. Naturally occurring mica, oil slicks and soap bubbles all display to some degree a level of iridescence. This iridescence or change in color as the viewing angle is changed, is a direct result of light reflecting from parallel interfaces from single or multilayer thin films. In general, the larger the refractive index difference across the interface(s) the greater the color effect. Color results from the interference of light. Maximum destructive reflective light interference occurs when the thickness of the layers are an odd number of quarter waves whereas maximum constructive light interference occurs when the thickness of the layers are an even number of quarter waves. Iridescent coatings which are called nacreous pigments when broken into small platelets are described in U.S. Pat. Nos. 3,087,828 and 3,123,490. These nacreous pigments are composed of single layer or multilayers in the optical thicknesses ranging from 10-100 nanometers typically prepared by vacuum deposition processes. These nacreous pigments are white or silvery and have very low color saturation regardless of the viewing orientation. Color is due mainly to simple Fresnel light reflection, scattering and/or absorption. Within many applications, it is desirable to achieve greater color saturation, i.e., chroma, than that can be achieved with nacreous pigments. In addition to chroma, there is a need for different colors and different color combinations which can be generated by using optically variable pigments. There is a particular need for such a multitude of colors for numerous anti-counterfeiting applications as well as other applications.